"The filters in your HVAC system are the first and most effective measure in protecting your heating and cooling system. Any HVAC specialist or home improvement guru will tell you, change your filters often. Keeping your system in fresh filters will greatly extend its life expectancy and this small bit of preventative maintenance can save you money on repairs down the road, not to mention help keep up the air quality in your home.
Although some products claim to be effective for ninety days or even longer, most industry experts recommend changing your filters every thirty days. In any case, you should inspect your filters regularly, as often as once a week, as the age of your HVAC system and the level of usage will determine how much wear and tear a filter suffers.
You know it is time to change your filters when it is coated in thick dust, usually gray or black. When a filter is clogged up like that, not only is it no longer doing its job, it is decreasing the efficiency of your heating system as the trapped dust absorbs heat. Plus, it also creates a serious fire hazard."